Since carbon fibers have a higher specific strength and specific modulus as compared with other fibers, the carbon fibers have also been widely applied, as reinforcing fibers for composite materials, to general industrial uses such as for automobile, civil engineering and architecture, compressed container and wind turbine blade, in addition to conventional sporting goods applications and aerospace space applications, and both further improvement in productivity and enhanced performance have been highly demanded.
Among the carbon fibers, a polyacrylonitrile (which may be hereinafter abbreviated as PAN)-based carbon fiber, which is most widely used, is industrially produced in such a way that a spinning solution composed of a PAN-based polymer, which is a precursor of the fiber, is subjected to a wet spinning, dry spinning or a dry-wet spinning to obtain a carbon fiber precursor fiber (which may be hereinafter abbreviated as a precursor fiber), the carbon fiber precursor fiber is then converted to a oxidized fiber by heating under an oxidizing atmosphere at a temperature of 200 to 400° C., and the oxidized fiber is carbonized by heating under an inert atmosphere at a temperature of at least 1,000° C.
In order to obtain a high-performance carbon fiber, the tension or draw ratio of a fiber bundle is often set higher in the production process described above. However, as the draw ratio or tension is increased, generation of fuzz or fiber breakage occurs more often. When the generation of fuzz or fiber breakage occurs, the grade and quality are decreased, dropped fuzz or broken fibers is wound around a roller or deposited in a furnace, and more likely to damage a subsequent fiber bundle. Thus, for stable production, there is a problem that it is not possible to set a high draw ratio enough to obtain a high-performance fiber, and the production has to be handled at a temporizing draw ratio in the trade-off relationship. In particular, techniques have been proposed for aiming at stabilization of drawing by allocating a profile for drawing in accordance with the progress of a heat resistance imparting reaction in a oxidation step (see Patent Document 1 and Patent Document 2). However, these patent documents only present the selection of the temporizing draw ratio as described above, and fail to disclose any techniques for allowing high draw ratios to be set fundamentally in a oxidation step, and if the temporizing draw ratio as described above is selected to handle the production on the basis of the documents, fiber breakage is not able to be sufficiently reduced.
On the other hand, the improvement in productivity of the PAN-based carbon fiber has been examined in terms of any of making, oxidizing, or carbonizing carbon fiber precursor fibers. Above all, conventional techniques concerning improvement in productivity of precursor fibers have the following problem. More specifically, in producing precursor fibers, the productivity is subjected to constraints by the number of spinneret holes, the limit velocity of taking up coagulated fibers according to the properties of the PAN-based polymer solution, and the limited draw ratio (which may be referred to as a limited draw ratio) related to the coagulated structure (hereinafter, the property indicating the limit velocity of taking up coagulated fibers is referred to as spinnability). Specifically, for obtaining carbon fiber precursor fibers composed of a large number of single fibers, the conditions influencing the productivity have to be determined depending on how much the final producing-precursor-fibers velocity determined by the product of the spinning speed and the draw ratio is increased. More specifically, when the spinning speed is increased in order to improve the productivity, the drawability is decreased, and the production process is thus likely to be destabilized. On the other hand, when the spinning speed is decreased, the production process is stabilized while the productivity is decreased. Thus, there has been a problem that it is difficult to achieve both improvement in productivity and stabilization of the production process.
Since it is known regarding the problem described above that the spinning method has a significant influence on spinnability, an explanation will be given for each spinning method.
In a wet spinning method, a spinning solution is extruded from a spinneret hole in a coagulation bath to the coagulation bath. Thus, coagulation proceeds immediately after the spinning solution is extruded from the spinneret hole. Therefore, the substantial draft ratio at spinning is increased with increase in taking up velocity. The increase in draft at spinning causes fiber breakage at a spinneret surface, and there is thus a limit on the increase in taking up velocity.
In contrast, in a dry-wet spinning method, a spinning solution is extruded once into the air (air gap), and then introduced into a coagulation bath, and yarn is thus mostly drawn at a low tension in the air gap. Therefore, it is known that the substantial draft at spinning in the coagulation bath is reduced to increase the spinnability. For example, a technique has been proposed in which the polymer concentration of the spinning solution is controlled to reduce the viscosity of the spinning solution, promote the handleability in filtration operation, and improve the draft ratio at spinning, which is the ratio between the velocity of taking up fibers in the coagulation bath and the extrusion velocity of a spinning solution from a spinneret (see Patent Document 3). According to this proposal, while an improvement effect is recognized as the draft ratio at spinning is 10, the draft ratio at spinning is only increased by the increased hole diameter of the spinneret. More specifically, the increased hole diameter of the spinneret slows down the linear extrusion velocity to increase the draft ratio at spinning. However, it is not possible to improve the productivity of the precursor fiber, because the spinnability is not improved only by the increase in draft ratio at spinning.
While a technique has been proposed in which the draft ratio at spinning is set at 5 to 50 by using a high-velocity spinning solution and providing a specific air gap (see Patent Document 4), this proposal relates to an acrylic fiber for closing, in which the number of substantial single fibers forming a fiber bundle is as small as 36, and is thus not suitable for carbon fibers obtained by oxidizing and carbonizing fiber bundles composed of a large number of, several thousand to hundreds of thousands of single fibers.
More specifically, in each of the conventionally known methods, the effect of improvement in productivity is limited. Accordingly, there has been demand for techniques for improving the productivity of carbon fibers, which can increase both the spinnability and the limited draw ratio even in the case of fiber bundles composed of a large number of single fibers, and further can suppress the generation of fuzz or fiber breakage which decreases the quality and grade and further the stability in production even in the case of using oxidation conditions including a high draw ratio.
The fact that little fuzz as carbon fibers have not only the advantage of process stability in a prepreg production process and a composite production process, but also the high incidence of the composite compressive strength for a molded body molded with the use of the carbon fibers, because fiber misalignment due to fuzz and the like can be reduced. The meaning of achieving carbon fibers with little fuzz is significant, because the compressive strength is an important index for material design in the design of composites.
The cause of such fuzz is considered to be partly a lattice defect of a hexagonal carbon layer. It is possible in principle to evaluate the lattice defect of the hexagonal carbon layer with the use of a Raman spectrum. While there have been conventionally a lot of study examples for the evaluation of carbon fibers with the use of a Raman spectrum (see Patent Documents 5 and 6), many studies regarding crystallite structures have been carried out whereas no discussion has been made regarding lattice defects. In addition, in the techniques disclosed in these documents, the crystallite structure of the carbon fibers is only controlled on the basis of the evaluation, and no lattice defect is controlled. Therefore, while the techniques for improving the average values for properties have been disclosed, no technique for improving variations in properties has been disclosed.
In addition, the cause of fuzz will be considered while focusing attention on carbon fiber bundles. Since fuzz is generated by breakage of weak fibers, the magnitude of variation in strength is related to the number of fuzzes. The variation in strength for carbon fibers is indicated by Weibull parameters (a Weibull shape parameter and a scale parameter) in many cases, and the variation in mechanical properties of carbon fibers as a resin impregnated strand is slightly improved in the case of a composite material formed with the use of carbon fibers which have the same mechanical properties of carbon fibers as a resin impregnated strand and is different in Weibull shape parameter. However, no example of significant improvement in average value for the physical properties is known. For example, carbon fibers have been proposed which have a single fiber tensile strength distribution specified by a Weibull shape parameter (see Patent Documents 7 and 8). In Patent Document 7, in order to suppress fuzz which may be generation in a graphitization process, the single fiber tensile strength distribution for carbon fibers which have a tensile modulus of carbon fibers as a resin impregnated strand of 305 GPa before the graphitization process is controlled to be narrow (have a Weibull shape parameter of 5 to 6). In accordance with this technique, the improvement of the tensile modulus of carbon fibers as a resin impregnated strand leads to brittle fracture morphology, and stress concentration is thus more likely to occur. Thus, the properties are more likely to be affected by defects, resulting in a decrease in Weibull shape parameter. In addition, in Patent Document 8, carbon fibers are proposed which are suitable for filament taking up processing and excellent in opening properties. Patent Document 8 mentions that the cross sectional shape and surface morphology of fibers are made more appropriate, the passage through the process for processing is improved without a large amount of converging agent, and it is important to control the Weibull shape parameter to be 4 to 6 in order to the improved passage. However, the modulus is 270 GPa or less, and the balance between a high modulus and a narrow variation in single fiber strength has not been achieved.                Patent Document 1: Japanese Patent Application Laid-Open No. 62-257422        Patent Document 2: Japanese Patent Application Laid-Open No. 58-186614        Patent Document 3: Japanese Patent Application Laid-Open No. 64-77618        Patent Document 4: Japanese Patent Application Laid-Open No. 11-107034        Patent Document 5: Japanese Patent Application Laid-Open No. 3-180514        Patent Document 6: Japanese Patent Application Laid-Open No. 9-170170        Patent Document 7: Japanese Patent Application Laid-Open No. 4-222229        Patent Document 8: Japanese Patent Application Laid-Open No. 2002-266173        
It could thus be helpful to provide a process for producing a precursor fiber for a high-grade carbon fiber which is less likely to cause fuzz without imparting the productivity. In addition, it could be helpful to provide a carbon fiber precursor fiber in which a high-grade and high quality carbon fiber can be produced without imparting the productivity while suppressing fuzz and fiber breakage even under oxidation-carbonization conditions including a high tension or draw ratio.